Performance of stone columns with circumferential nails

This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the improvement in stiffness, load carrying capacity and resistance to bulging of stone columns installed in soft soils. A new method of reinforcing the stone columns with vertical nails installed along the circumference of the stone column is suggested for improving the performance of these columns. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. It is found that stone columns reinforced with vertical nails along the circumference have much higher load carrying capacity and undergo lesser compression and lesser lateral bulging as compared to conventional stone columns. The benefit of vertical circumferential nails increases with increase in the diameter, number and depth of embedment of the nails. The improvement in the performance of stone column was found to be more significant, even with lower area ratio. It is found that reinforcing stone column with vertical circumferential nails at the top portion to a depth equal to three times the diameter of stone columns, will be adequate to prevent the column from excessive bulging and to improve its load carrying capacity substantially.

Stone columns have become a widely used method of increasing bearing capacity of soft soils. This study investigates floating stone columns with and without encasement in soft soils. Although the stone columns in single-layered soil have been studied extensively, stone columns constructed in a base of varying soil layers are not fully understood. In the present study, the behavior of both single-layered soft soil and layered soil consisting of loose sand overlaying the soft soil was investigated by using small-scale laboratory pilot tests. The bearing capacity of soft soil was improved in all cases of stone column application. The contribution of stone columns on the bearing capacity of soft soil was presented by the term bearing improvement ratio (BIR). With a non-encased stone column in single-layered soft soil, the BIR was about 3.3-fold and with geotextile encasement in the same soil, the improvement ratio increased to 3.4-fold. For a non-encased stone column in layered soils, the BIR was about 2.0-fold and with geotextile enhancement in the same soil, this improvement ratio increased to 4.0-fold. The inclusion of geotextiles resulted in improved bearing capacity by distributing the induced stresses over larger areas. The maximum bulging of non-encased stone column in single-layered soft soil was observed at the depth of 1.5 times the original diameter of the stone column from the top, whereas for encased stone column in single-layered soft soil, the maximum bulging was transferred to a depth of 3.0 times the original diameter of the stone column.

Stone columns (or granular piles) have proven to be ideal ground reinforcement for supporting flexible structures such as embankments and storage tanks. Stone columns installed in very soft soils will have very low lateral confinement: hence they undergo excessive bulging, leading to undue settlement and limited load-carrying capacity. In these situations, the strength and stiffness of the stone column can be enhanced by encasing the individual stone columns with a suitable geosynthetic. The encasement improves load transfer to deeper depths of soil. This paper investigates the qualitative and quantitative improvement of load capacity of individual encased stone columns through laboratory model tests. These tests were performed in a rigid unit cell that represents the stone column and the soil within the contributary area around the stone column. The load tests indicated a clear improvement in the load capacity of the stone column due to encasement. Encasement with geosynthetics having higher modulus resulted in stiffer response. The effect of encasement was found to decrease with increase in the diameter of the stone column. The improvement in the performance of stone columns was found to be significant, even with partial encasement.

Nowadays, some projects have to be constructed in areas having thick layers of soft clay.These soft soils with low shear strength and high voids ratio, lead to excessive settlements even if subjected to low vertical or lateral loads.For this reason, soft clays are considered problematic soils for foundation purposes.Several improvement techniques have been done to enhance such clay.Stone columns have been used to improve soft soils by increasing its carrying capacity and reducing the settlement.In this paper, a numerical study on seismic behavior of stone column in soft clay subjected to earthquake loading has been performed.A two-dimensional plane strain program PLAXIS (dynamic version) is used for the present numerical modeling.A series of modeled stone columns were simulated with different diameters and spacing between columns.Also, the influence of geotextile encasement on the performance of stone columns, foundation systems in soft clay is investigated and compared with the behavior of ordinary stone columns without casing.The results showed that the ordinary stone column with small spacing and larger diameter has a greater bearing capacity and give a smaller settlement, compared to the column with large spacing.In addition, the geotextile encasement for stone column can be provided a significant increase in stone column capacity as well as a huge reduction in settlement is considered with increasing the encasement strength.

Stone columns are widely used as an effective and environmental friendly improvement method for increasing the load-carrying capacity of soft clay soils. In very soft clay soils, reinforced stone columns are used because of the lack of the lateral confinement created by the surrounding soil. To provide lateral additional confinement, geosynthetics are usually used. This study intends to evaluate the use of vertical steel bars and horizontal steel discs as an alternative way to geosynthetics to investigate the effect of reinforcement on the footing load-carrying characteristics. Therefore, some large-scale laboratory tests were conducted on stone columns with diameters of 80 and 100 mm and a length to diameter of 5. The results show that changing the arrangement of the bars to a higher stiffness leads to increase in load-carrying capacity. Reinforcing the full-length of the stone columns with the bars in comparison to half-length reinforced has significant influence in capacity. However, in the case of horizontal discs, this increase is negligible. Also by decreasing the space of the discs, load-carrying capacity increases. Moreover, the performance of the vertical reinforced stone column seemed to be better than the horizontal reinforced stone column. The increase of load-carrying capacity in reinforced stone columns with vertical bars or horizontal discs is higher than geotextile reinforcement in the same conditions.

The stone columns are increasingly being used as a soil improvement method for supporting a wide variety of structures (such as road embankment, buildings, storage tanks etc.) especially built on soft soil. Soil improvement by the stone column method overcomes the settlement problem and low stability. Nevertheless, stone column in very soft soils may not be functional due to insufficient lateral confinement. The required lateral confinement can be overcome by encasing the stone column with a suitable geosynthetic. Encasement of stone columns with geogrid is one of the ideal forms of improving the performance of stone columns. This paper presents the results of a series of experimental tests and numerical analysis to investigate the behavior of stone columns with and without geogrid encasement in soft clay deposits. A total of six small scale laboratory tests were carried out using circular footing with diameters of 0.05 m and 0.1 m. In addition, a well-known available software program called PLAXIS was used to numerical analysis, which was validated by the experimental tests. After good validation, detailed of parametric studies were performed. Different parameters such as bearing capacity of stone columns with and without geogrid encasement, stiffness of geogrid encasement, depth of encasement from ground level, diameter of stone columns, internal friction angle of crushed stone and lateral bulging of stone columns were analyzed. As a result of this study, stone column method can be used in the improvement of soft ground and clear development in the bearing capacity of the stone column occurs due to geogrid encasement. Moreover, the bearing capacity is effected from the diameter of the stone column, the angle of internal friction, rigidity of the encasement, and depth of encasement. Lateral bulging is minimized by geogrid encasement and effected from geogrid rigidity, depth of encasement and diameter of the stone column.

Stone columns are widely used and generally considered to be one of the most cost-effective and environmental-friendly soil improvement technique for highways and embankments. They are also used as drainage to reduce the consolidation period, which accordingly increases the bearing capacity, reduces settlement, and reduces the liquefaction potential. Current design theories used to estimate the bearing capacity of a group of stone columns are based on the unit cell or homogenized material concepts, which neglect the effect of the column interactions and installation technique. This thesis therefore presents an experimental investigation, together with numerical modelling, to examine the performance of a single stone column and group of stone columns subjected to vertical loading. An analytical model is developed to capture the effect of an arrangement of stone columns and the mode of failure within a column and the surrounding soft clay material. A single stone column and a group of stone columns were investigated in a large-scale experimental set-up. The testing program was divided into four steps: (a) filling the testing tank with the clay, (b) installing the stone columns in the clay bed, (c) extracting samples of the reinforced soil (a block of stone columns surrounded by the soft clay loading), and (d) testing the samples in a triaxial apparatus. The results showed that the mode of failure of the reinforced soil depends on the column spacing and the strength of the column materials and the surrounding soil. Numerically, a 3-D finite element model was developed to examine the influence of the governing parameters on the bearing capacity of the group. The model was validated against experimental results from this study and results available in the literature. The numerical model was used to simulate the actual driving process during installation of the columns. The model was then used to predict the actual failure plane under a rigid footing reinforced by stone columns for a given geometry/soil condition. An analytical model was developed utilizing the actual failure plane deduced from the numerical model to develop a theory to predict the bearing capacity of the reinforced soil. The theory developed was validated against the results obtained from the numerical model and results reported in the literature.

Influence of encasement length and geosynthetic stiffness on the performance of stone column: 3D DEM-FDM coupled numerical investigation

The application of stone column technique for improvement of soft soils has attracted a considerable attention during the last decade. However, in a very soft soil, the stone columns undergo excessive bulging, because of very low lateral confinement pressure provided by the surrounding soil. The performance of stone column can be improved by the encapsulation of stone column by geosynthetic, which acts to provide additional confinement to columns, preventing excessive bulging and column failure. In the present study, a detailed experimental study on behavior of single column is carried out by varying parameters like diameter of the stone column, length of stone column, length of geosynthetic encapsulation and stiffness of encapsulation material. In addition, finite-element analyses have been performed to access the radial deformation of stone column. The results indicate a remarkable increase in load carrying capacity due to encapsulation. The load carrying capacity of column depends very much upon the diameter of the stone column and stiffness of encapsulation material. The results show that partial encapsulation over top half of the column and fully encapsulated floating column of half the length of clay bed thickness give lower load carrying capacity than fully encapsulated end bearing column. In addition, radial deformation of stone column decreases with increasing stiffness of encapsulation material.

ABSTRACT: The use of stone columns is popular as a ground-reinforcing technique for supporting flexible structures on soft to very soft soils. When a very soft soil improved by stone columns is loaded, the stone columns undergo excessive bulging, because the very low lateral confinement provided by such a soil gives rise to a very low failure stress of the composite ground. The performance of stone columns in such conditions can be improved either by encasing them with geosynthetics or by placing horizontal strips of geosynthetics within the columns at regular intervals. In the present study, model tests have been carried out on short, floating and fully penetrating single columns with and without reinforcement to evaluate the relative improvement in the failure stress of the composite ground due to different types of reinforcement. A comparative evaluation of the column shape on exhumation reveals subtle differences in failure mode due to the different configurations and types of reinforcement. Whereas for the end-bearing stone columns encasement is clearly the best method, for the floating columns there is no significant difference between the performance of the horizontal strip reinforcement and that of the encasement. For end-bearing columns a geogrid is the best type of geosynthetic to use for both types of reinforcement, but for floating columns the best types of geosynthetic are geogrid as the horizontal strip and geotextile as the encasement.

Stone columns are often designed using the conventional framework of saturated soils, ignoring the influence of in-situ unsaturated soil conditions. Such an approach contributes to unrealistic and, in certain scenarios, over-conservative designs. The key objective of this paper is to quantify the influence of matric suction on the confining support offered by the surrounding soil towards stone columns. In addition, how this approach can be implemented through a numerical technique is also presented and discussed. The numerical analysis suggests that the load-carrying capacity of stone columns increased with an increase in the matric suction in the boundary effect, and the transition zone. However, the contribution of matric suction towards load-carrying capacity starts reducing from the residual zone of saturation. Furthermore, the performance of stone columns in unsaturated soils is strongly associated with the area replacement ratio. The results of the study are promising towards developing procedures that can be used in the rational design of stone columns in unsaturated soils.

Bearing capacity is very important in geotechnical engineering, which depends on factors such as footing shape, stress distribution under footing and failure mechanism of soil. One of the methods for improving the bearing capacity and reducing settlement in soft and fine soils is adding column like elements to soil which called stone column. In this research using model tests and numerical modeling, the effect of existence and location of stone column on bearing capacity of strip footing near soft clay slope is studied. In fact, reinforced and unreinforced stone columns in different locations are added to slope and the effect of them on load-settlement behavior of strip footing rested on top of the slope was investigated. Group stone columns are also studied, and efficiency of them is investigated. Also numerical modeling is carried out with Plaxis 3D Foundation program, and finally the results of experimental and numerical modeling were compared. Results show that reinforcing clay slope with stone column in all situations leads to increase in bearing capacity of strip footing. Moreover, reinforcing stone columns with encasing cause better performance of stone columns and increase in bearing capacity of footing compared with the same unreinforced stone columns. The maximum effect of stone column on bearing capacity of strip footing occurs when the stone column is located beneath the footing, and with increase in distance between the column and footing, the bearing capacity of footing is decreased.

Chapter Fifteen - Experience in using sensitivity analysis and ANN for predicting the reinforced stone columns’ bearing capacity sited in soft clays

Short-term and long-term behavior of geosynthetic-reinforced stone columns

Chapter 22 Implementation and performance of stone columns at Penny's Bay reclamation in Hong Kong